The most detailed image released so far shows a tiny world riddled with ancient pockmarks, but with great variations of surface brightness. Phoebe in general is very dark, but close inspection revealed areas so bright they're washed out in the picture.
"What spectacular images," Carolyn Porco, Cassini Imaging Team leader at the Space Science Institute in Boulder, Colorado, said in a statement. "So sharp and clear and showing a great many geological features, large and small. It's obvious a lot of new insights into the origin of this strange body will come as a result of all this."
Phoebe is about 137 miles (220 kilometers) wide, or roughly one-fifteenth the diameter of Earth's Moon. Phoebe orbits Saturn backward compared to the planet's rotation. That plus its dark surface has long had scientists speculating that it might be an object captured from the Kuiper Belt, a region of icy bodies beyond Neptune.
Though it is too early to say, researchers believe closer inspection of the pictures of Phoebe could reveal this to be the case, and could also tell them more about the early solar system.
"Phoebe is a heavily cratered body," said Torrence Johnson, Cassini imaging team member at NASA's Jet Propulsion Laboratory, Pasadena, Calif. "We might be seeing one of the chunks from the formation of the solar system, 4.5 billion years ago. It's too soon to say."
The highest resolution image so far released shows a crater near the right hand edge with bright rays that extend from its center. This suggests that dark material coats the outside of what may be a predominantly icy body, scientists said. Kuiper Belt Objects, more so than asteroids, are thought to be made up largely of ice.
The large craters has also led to speculation that Phoebe, the largest of Saturn's outer moons, might be the parent of the other, much smaller backward-orbiting outer moons of Saturn. "Looking at those big 50 kilometers (31 mile) craters, one has to wonder whether their impact ejecta might be the other tiny moons that orbit Saturn on paths much like Phoebe's," said Joseph Burns, an imaging team member and professor at Cornell University.
Craters on Phoebe are thought to be the result of collisions with smaller objects, some perhaps up to 328 feet (100 meters) wide.
During Cassini's planned four-year tour it will orbit Saturn 76 times and execute 52 close encounters with seven of Saturn's 31 known moons. It may discover other moons not yet spotted within Saturn's ring system. But this will be the last close look at Phoebe during the mission.
Cassini came within about 1,285 miles (2,068 kilometers) of the dark moon. The spacecraft imaged Phoebe and took radar and other readings. Several hours later it turned to point its antenna to Earth and send pictures and data back.
Cassini sped by the moon at a relative speed of 13,900 mph (20,900 kilometers per hour) relative to Saturn.
The last visit to Phoebe was with Voyager 2 in 1981. That flyby was 1,000 times farther from the moon, however.
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